The president has repeatedly spoken publicly about his cancer battle but has never specified what type of cancer it is. And his government has released few specifics, fueling widespread speculation about Chavez's health and political future.

Chavez had surgery in 2011 to remove a cancerous tumor and has undergone further surgery and radiation in Cuba since. He declared himself cancer-free in July.

Health rumors dogged Chavez on the campaign trail this year but didn't stop him from winning re-election in October.

Venezuela's constitution specifies that when a president dies, the vice president assumes the presidency until new elections can be held.

Chavez called for voters to take things a step further.

"My firm opinion, as clear as the full moon -- irrevocable, absolute, total -- is ... that you elect Nicolas Maduro as president," Chavez said, waving a copy of the Venezuelan constitution as he spoke. "I ask this of you from my heart. He is one of the young leaders with the greatest ability to continue, if I cannot."

It was the first time Chavez -- who looms larger than life in Venezuela and in Latin American politics-- had spoken publicly about the possibility of a successor.

"This is huge. He could have said something indirectly. He could have said something like, 'We'll have to see. Let's talk about it when the time comes,'" said Javier Corrales, a professor of political science at Amherst College in Massachusetts. "He switched from being very evasive to very articulate. That must have been the result of a major change in health for the worse."

In a report published Tuesday, one expert on the region said some observers think that Chavez's health problems are being used for political purposes.

"One speculative theory holds that Chavez's health problems have been serious but not life-threatening -- trumped up to focus attention on him, generate sympathy during this year's election campaign, and now may even be a way of boosting Venezuela's bond sales," wrote Stephen Johnson, director of the Americas program at the Washington-based Center for Strategic and International Studies.

Keeping the focus on the Venezuelan president could also be a political advantage going into gubernatorial elections, which are scheduled for Sunday, Johnson said. But other signs point to more serious health problems, he said, including a decrease in Chavez's public appearances in recent months.

"Reports may or may not be true since there is no independent, reliable information source other than the president, who decides what he wants to divulge," Johnson said.